Commentaries of the Four Authorized Vaisnava Sampradayas

The reason Lord Krishna emphasises its better to perform one's own duty
imperfectly then another's duty perfectly is because the defective
performance of one's own duty is superior in merit to the performance of
another's duty perfectly executed. This is to reinforce in Arjuna the
futility of concocting any notions that living on alms as a mendicant
beggar which is the duty of Brahmins is superior then his duty of fighting.
Begging is not the path for him to take as it is inferior in terms of merit
then fighting in battle and dispatching the enemy which is Arjunas natural
duty. By following one's own natural path no sin is incurred.

Brahma Vaisnava Sampradaya:

Madhvacarya

There is no commentary for this verse.

Sri Vaisnava Sampradaya:

Ramanuja

Ramanuja's Commentary

To adhere to one's own natural path of dharma or righteousness is what is
being emphasised by Lord Krishna here. It is not as if one must follow the
path of another. Everyone is entitled to perform those activities that are
appropriate for their rank and station in life in constituting their
propitiation to the Supreme Lord; but one must eliminate all sense of
doership and desire for rewards. Such karma yoga or actions facilitating
communion with the Supreme absolute expressed in visible activities which
conform to the inherent nature of the individual manifest easily and
naturally. Thus karma yoga performed in this manner is factually one's own
path of dharma albeit destitute of merit. By another's dharma is meant
jnana yoga or facilitating communion with the Supreme absolute by knowledge
which depending on time and circumstance may or may not be helpful.
Perfection in jnana yoga demands the consistent ability to completely
control the mind and conquer the senses which is a lifetime endeavour and
fraught with danger of lapses in constancy. So karma yoga is more
fortuitous as the risks are minimal and chances for succeeding are greater
even though jnana yoga is on a superior platform. A jiva or embodied being
ensconced in a physical body with an appropriate mind and senses finds it
normal to act in accordance with the natural impulses instigated by the
senses. This is karma yoga and fulfilling activities in this manner if they
do not contradict the injunctions of the Vedic scriptures or disregard the
prohibitions thereof incur no sin, for actions speak for themselves.
Whereas in jnana yoga once control of the mind and mastery of the senses
has been achieved they must be kept controlled and mastered otherwise one
will fall down from their position and subjected again to mundane desires
and the influence of objects the senses which propels one to sin and locks
one in samsara the perpetual cycle of birth and death. So by clearly
elucidating the fact that karma yoga is indeed safer and more sure of
success, Lord Krishna further corroborates what He previously had
promulgated in chapter three.

Kumara Vaisnava Sampradaya:

Nimbaditya

Kesava Kasmiri's Commentary

Because the natural propensity inherent within is always accurate it
behoves one to perform prescribed Vedic activites unto the Supreme Lord as
this leads to the highest perfection. Such activities should be performed
consistently for securing one's own best interests. This Lord Krishna
alludes to with the sreyan meaning better for it is better to perform one's
own duty with the intention to propitiate the Supreme Lord even if devoid
of merit or deficient in some way; it is still superior to performing the
duties of another even if done exemplary. The performance of duties not
sanctioned for one in the Vedic scriptures is risky and fraught with danger
is susceptible to sin; but in the performance of one's own natural
prescribed duty no sin is incurred.